1. Field of Invention
The present invention relates to a photolithographic process in semiconductor manufacturing. More particularly, the present invention relates to a photolithographic process capable of preventing the rounding of the corners in a pattern.
2. Description of Related Art
Following the rapid increase in the level of integration, integrated circuits are now designed with decreasing feature dimensions. In the fabrication of semiconductors, one very important manufacturing step is photolithographic process. All metal-oxide semiconductor (MOS) devices related processes such as thin film patterning or dopants implantation are conducted by performing photolithographic processes. Ultimately, the fabrication of integrated circuits with a line width smaller than 0.18.mu.m depends on the future development of photolithography. To reduce size of semiconductor devices, resolution of photomask in photolithographic process must be increased. Recent development in this direction includes optical proximity correction (OPC) and phase shift mask (PSM).
Optical proximity correction is at present one of the principle methods of eliminating critical dimension deviation due to proximity effect. Proximity effects occur when a light beam passing through a photomask with a pattern thereon is projected onto the surface of a silicon chip. In the process, the light rays may be diffracted by the photomask so that a portion of the light rays may diverge. Furthermore, some of the light passing into the photoresist layer above the silicon chip may be reflected by the semiconductor substrate of the silicon chip to cause interference. Hence, a portion of the photoresist layer may be repeatedly exposed leading to undesirable variation in photoresist exposure.
FIG. 1A is a top view of a conventional photomask with a trench array pattern for manufacturing trench capacitors. FIG. 1B is a top view of the pattern on a silicon wafer after photolithographic processing with the photomask shown in FIG. 1A.
As shown in FIG. 1A, a trench pattern 102 is formed on a photomask 100. For example, if the trench pattern 102 is a trench pattern for forming trench capacitors, width 102a and length 102b of separation between neighboring trenches are different. In other words, pattern density in the horizontal and the vertical directions are different and hence corresponding intensity and strength of exposure requirements are different. Due to proximity effect, differences in the edge length and edge width of the trenches will limit the process window of the photolithographic process.
In addition, due to proximity effect, the corners of pattern is somewhat rounded leading to pattern distortion when the trench pattern 102 on the photomask 100 is transferred to the silicon wafer 104 to form an opening pattern 106.
To reduce proximity effect, subsidiary patterns are often formed around the trench pattern on the photomask. However, as device dimensions shrink and the level of integration increases, line width becomes increasingly narrow and there is limited area around the device pattern for forming a subsidiary pattern. Consequently, cost of fabricating the photomask is increased. Moreover, as line width of a pattern shrinks, the limited resolution of a stepping machine will further reduce the process window for forming an opening pattern on a silicon wafer using photolithographic process.